Driving under the influence of drugs (DUID) is a worldwide problem. Several countries have adopted DUID legislations which prove their deterrent effect and impact on road safety. However, the use of new psychoactive substances (NPS) and prescription drugs is not known, as the applied roadside screening tests have not yet been adapted for these compounds. In this study, 558 blood samples obtained during roadside controls in Belgium (January to August 2015) after a positive Drugwipe 5S test and 199 oral fluid (OF) samples obtained from negatively screened test pads were analyzed. The NPS positivity rate was 7% in blood, while it reached 11% in OF. NPS detected were: diphenidine, ketamine, 4-fluoroamphetamine, 2-amino-indane, methoxetamine, alpha-PVP, methiopropamine, a mix of 5-MAPB/5-EAPB, TH-PVP, mephedrone, methedrone, 4-methylethylcathinone, 5-MeO-DALT, 4-Acetoxy-DiPT, AB Fubinaca, FUB-JWH018, JWH020, trifluoromethylphenylpiperazine, and ethylphenidate. Moreover, 17% of blood samples (and 5% of OF) contained an analgesic drug, 10% (0.5%) a benzodiazepine/hypnotic, 5% (2%) an antidepressant, 2% (3%) an antipsychotic, 2% an antiepileptic drug, and 1% methylphenidate. The presence of NPS in the young (and predominately male) DUID population is proven. Furthermore, a high level of poly-drug use including combinations of NPS, licit, and drugs of abuse was observed. Further research concerning the development of on-site NPS detection techniques should be established. Meanwhile, the effects of combined drug use on driving ability and the physical/psychological signs after NPS use should be performed to improve the on-site DUID detection of NPS by police officers, so they can engage in blood sampling for a general unknown screening.